To the inputs of electronic devices, particularly measuring instruments such as oscilloscopes, are often applied voltages having both a DC and an AC component. In many such instances it is only the AC component which is relevant to the measuring operation while the magnitude of the DC component is of no concern. To eliminate the DC component, a DC voltage is, in the input circuit of the electronic device, added to the input signal, said DC voltage having the same absolute magnitude as the DC component but the opposite polarity
It is well known to use an analog summing amplifier with two inputs for this purpose. The input signal consisting of the DC and AC components is applied to one of the inputs, while the compensating DC voltage is applied to the other input. At its output, the amplifier supplies only the AC component of the input signal. In many cases, however, the absolute magnitude of the DC component of the input voltage is substantially greater than the amplitude of the AC component. Although only the AC component is of importance, the dynamic range of the summing amplifier must in such cases be adapted to the overall magnitude of the input signal. Moreover, it is necessary that the summing amplifier have a linear characteristic over the entire dynamic range and that any spurious signals be small not only with respect to the DC component but also with respect to the AC component of the input voltage.
The dynamic range of the summing amplifier can, of course, be reduced by connecting a voltage divider before it, but this reduces the AC component of the input signal in the same proportion as the DC component.
This imposes an a priori maximum limit on the voltage division ratio.